Rayleigh-wave attenuation across the conterminous United States in the microseism frequency band

Fabrizio Magrini,Fabio Cammarano,Lucia Gualtieri,Lapo Boschi,Vedran Lekić

doi:10.1038/s41598-021-89497-6

Abstract

Mapping variations in the attenuation of seismic energy is important for understanding dissipative mechanisms in the lithosphere, and for modeling ground shaking associated with earthquakes. We cross-correlate ambient seismic signal recorded across the EarthScope Transportable Array in the 3–15 s period range. We apply to the resulting cross correlations a new method to estimate lateral variations in Rayleigh-wave attenuation, as a function of period, beneath North America. Between 3 and 6 s, our maps are dominated by a strong eastward decrease in attenuation. This pattern vanishes at longer periods, confirming early observations based on regional earthquakes. Attenuation maps and phase-velocity maps are anti-correlated at periods between 3 and 6 s, but the anti-correlation is also largely lost at longer periods. This corresponds to the attenuation coefficient decreasing with period more rapidly in the west than in the east, while the change in phase velocity with period is more uniform across the continent. Our results point to a transition in the properties of upper-crustal materials with depth, probably related to the closure of fluid-filled cracks and pores, and imply that measures of attenuation from seismic noise carry significant information on crustal rheology.

Highlights

The crust is the most heterogeneous region of our planet, and its structure is the integrated result of magmatic, erosive, depositional, and tectonic processes over billions of years
Seismic surface waves are naturally sensitive to dissipation; it is known that, in addition to geometrical spreading, the amplitude of a surface wave decays with epicentral distance according to the factor e−α, where α is usually referred to as “attenuation coefficient”, or “attenuation”7. α changes with frequency and location
We used all the available seismic data from the transportable component of the USArray, consisting of over 400 broadband seismometers deployed in 1600 different locations across the United States and part of Canada

Summary

Introduction

The crust is the most heterogeneous region of our planet, and its structure is the integrated result of magmatic, erosive, depositional, and tectonic processes over billions of years. By nonlinear inversion[13,14,15] of the data set compiled, we retrieved 440 measurements (one per sub-array) of the frequency-dependent Rayleigh-wave attenuation coefficient α (Fig. 1).

Results

Conclusion